High frequency electron discharge device with temperature compensated gap control means



Dec. 7, 1965 N. E. OAKS 3,222,565

HIGH FREQUENCY ELECTRON DISCHARGE DEVICE WITH TEMPERATURE COMPENSATED GAP CONTROL MEANS Filed July 12, 1963 I INVENTOR NORMA N E. OAKS ATTORNEY United States Patent M HIGH FREQUENCY ELECTRON DISCHARGE DE- VICE WITH TEMPERATURE COMPENSATED GAP CONTROL MEANS Norman E. Oaks, Woodside, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed July 12, 1963, Ser. No. 294,559 8 Claims. (Cl. 3155.48)

This invention relates to high frequency electron discharge devices and more particularly to temperature compensation and cooling means for such devices.

As a major problem encountered in high frequency electron discharge devices such as, for example, fixed tuned and tunable klystron oscillators and/ or amplifiers is the diificulty in maintaining frequency stability during operating conditions. The problem of frequency drift has a primary causal factor in the heating of the cavity resonator parts with consequent expansion of the cavity volume and/ or change in gap spacing due to expansion of drift tube elements including the drift tube header or diaphragm support members.

Generally, when a material such as copper is utilized for the drift tubes, support headers and tube main body, thermal expansion of these members will cause an increase in cavity volume and thus an increase in cavity inductance and a resultant decrease in the resonant frequency of the cavity. This inductive effect may be small in comparison to the frequency changeeffected by the variation in the interaction gap between drift tubes. This gap spacing will decrease due to thermal expansion'and elongation of the drift tubes with a resultant increase in interaction gap capacitance producing a further decrease in the resonant frequency of the cavity which may be greater than that due to the inductance change.

In either case, however, compensation is easily effected by controlling the gap spacing by exerting an axial force on the cavity header supporting one of the cavity drift tube members. The basic technique for such control through the use of materials having diverse temperature co-efficients of expansion is disclosed incopending US. Patent Application Serial No. 85,090, filed January 26,

1961, now US. Patent 3,117,251, issued January 7, i964,v

and disclosed and claimed in copending US. patent application Serial No. 294,558 filed July 12, 1963 which application is a continuation-in-part of SerialNo. 85,090, both of which applications are assigned to the same assignee as the present application.

The temperature compensation techniques set forth in the aforementioned copending applications required a body material surrounding the collector member which had a relatively high thermal coefficient of expansion with respect to the collector. However, the need for reducing the sensitivity of high frequency discharge devices to external magnetic fields required the use of magnetic materials in the surrounding body. Such materials as low carbon steel and cold rolled steel were found to provide the required magnetic shielding but were deficient with respect to temperature coefficient of expansion and thus didnot provide frequency stabilization. Therefore, another temperature compensation means was required, and the present invention provides a solution to the aforementioned problem.

This solution based on the generic concepts set forth and lairned in the aforementioned copending applications involved the provision of a separate high temperature coefficient of expansion temperature compensation means Patented Dec. 7, 1965 which is disposed between the collector and spaced surrounding body structure. A specific embodiment involves the use of a frustro-conical strut as the separate temperature compensation means. The frustro-conical strut is copper or another material having a higher thermal coefficient of expansion than the collector and body materials and provides an extremely simple solution to the aforementioned problem.

The present invention also teaches a novel means of cooling an electron discharge device wherein a low melting point alloy having a high thermal conductivity is positioned within a hollow region surrounding the collector portion of the device to facilitate conduction of heat from the beam collecting member to the collector cooling structure.

It is therefore an object of the present invention to provide an electron discharge device having improved temperature compensation and cooling.

A feature of the present invention is the provision of a temperature compensation means disposed between the beam collector and body portion of an electron discharge device and adapted and arranged whereby temperature compensation and thus frequency stabilization of the device is achieved.

Another feature of the present invention is the utilization of a frustro-conical strut for the temperature compensation means of the preceding feature.

. Another feature of the present invention is the provision of a low melting point alloy disposed around the collector portion of an electron discharge device to thereby enhance the cooling of said device.

Another feature of the present invention is the provision of an electron discharge device having a magnetic body and separate frequency stabilizing means.

These and other features of the present invention will become more clearly apparent upon a perusal of the following description taken in conjunction with the accompanying drawings wherein:

The sole figure in the case is a cross-sectional view of a two cavity klystron incorporating the novel temperature compensation and cooling features of the present invention. Disposed at one end of a predetermined line of circuit development and adapted and arranged to generate and direct an electron beam therealong is an electron gun assembly 1 having a focusing anode 2 and constructed in general as disclosed in US. patent application Serial No. 85,090 filed January 26, 1961. The tubedepicted in the figure herein is essentially identical to the tube depicted in FIG. 1 of the aforementioned U.S. Serial No. 85,090 with the exception of the collector structure.

A ceramic base 3 is vacuum sealed to metal sealing ring 4 which in turn is vacuum sealed to main body member or block 5. Plural cavities 6, 7 together with tuning diaphragm 8, tuning mechanism 9 and output window assembly 10 form the central portion of the "tube.

A central bore 12 extends along the length of the main body member as shown. Cavities 6, 7 are defined by headers or diaphragms 13, 14, 15 having re-entrant drift tube sections 17, 18, 19, respectively. Grid members 20 are disposed across each of the end portionsof re-entrant drift tube sections 17, 18, 19, as shown, to define interaction gaps 21, 22. A collector assembly 23, includes a collector tube member 24 disposed coaxially with the drift tube portion 19 and supported and fixedly attached on the one end thereof, as shown, to diaphragm or header 15. The electrons of the beam are collected on a baffle 25 of copper or the like disposed within and fixedly attached to tube 24 which functions to protect the pinchedoff exhaust tubulation 26 from direct electron bombardment and to dissipate the heat generated thereon by electron bombardment and direct same by conduction to the collector 24 where cooling is more conveniently applied. The tubulation is brazed or the like to the exterior end of tube 24, as shown, to form a vacuum seal.

The exterior end of the collector 24 has an external screw thread 27 on which is disposed a nut 28 which can bear down on the protruding end of the body when the nut 28 is tightened. Since the end diaphragm is flexible, it can be flexed by an axial force thereon which force is imposed by tightening nut 28 to thereby provide gap tuning means for the klystron. When the nut 28 is loosened, the diaphragm 15 will spring back slightly since it is typically stressed within its elastic limit.

A frustro-conical shaped expansion member or strut 29 having transverse flange 30 is disposed around collector tube 24 and brazed thereto at interface 31, as shown, and brazed to the surrounding body 5 at 32, as shown, leaving voids 33 and 34 on either side thereof. The strut 29 functions to provide the axial force for moving the end wall or diaphragm 15 to control the gap spacing.

In certain tube applications, such as in strong magnetic field environments, it is desirable to reduce the magnetic sensitivity of the tube. In such cases it is desirable to make the tube body 5 out of highly permeable material for magnetically shielding the beam from externally generated magnetic fields. Such highly magnetic permeable material includes, for example, a low carbon steel. Low carbon steel and other magnetic materials such as cold rolled steel have low temperature coefficients of expansion. Therefore, it is difficult to use the axial extension of the tube body as the high coefficient of thermal expansion member of the cavity temperature compensation structure described in the aforementioned US. Serial No. 85,090.

The frustro-conical strut 29 of the present invention, if made of a material having a higher coefficient of expansion than the body 5, and collector 24, such as copper, will provide the temperature compensation in the following manner. As the tube parts heat up during operation, the gap spacing will tend to decrease and the cavity volume increase. To compensate for both of these effects a minute overall increase in gap spacing, with respect to the original cold test circuit parameters, is required. The strut 29 provides this increase by expanding relative to the body 5 and collector 24, which is preferably made of a low coefficient of thermal expansion such as, for example, Kovar, Invar, steel, etc., and thereby producing an axial force component on the collector and flexible header 15 which will provide a minute increase in the gap spacing. This increase is preferably just suflicient to overcome the detuning caused by increase in the cavity volume and to precisely match the decrease in gap spacing caused by expansion of the drift tubes and associated headers. It is to be noted that the gap spacing may be controlled only to the extent necessary to compensate for the decrease in gap spacing and that the change in cavity volume may be ignored in instances where extreme frequency stability is not required.

Thus the present invention provides means for obtaining temperature compensation and thus frequency stabilization of an electron discharge device while simultaneously selecting a material for the body of the tube for its magnetic properties rather than for its coeflicient of expansion properties.

More eflicient heat transfer between the collector and the surrounds is obtained at increased collector dissipation levels as of 100 Watts c.w. by filling the spaces or voids 33, 34 between the collector 24 and body 5 with a low melting point alloy such as a bismuth-lead-tin-cadmium alloy known as Cerrobend manufactured by the Pasco Corporation having a melting point of C. or Cerrolo 117 which is a composition of bismuth-lead-tin-cadmium and indium also made by Pasco Corporation having a melting point of 48 C. The low melting point alloy provides excellent heat transfer between the collector and the body by conduction and convection whereby the heat may be removed from the body by air-cooling the tube or by mounting the tube on a heat sink. The employment of a liquid state heat transfer medium as described above is doubly advantageous in the present environment since a solid heat transfer medium could not be utilized since it would negate or render the temperature compensation mechanism of the present invention inoperative.

Since many changes could be made in the above construction and many apparently widely different embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high frequency apparatus comprising a metal body having a bore therein, electron gun means operatively connected thereto at the one end thereof, collector means operatively connected thereto at the other end thereof, an interaction gap formed by opposing drift tube sections disposed within said bore between said collector and gun means, and a frusto-conical strut disposed about said collector and operatively connected to said collector and said body, said frusto-conical strut being made of a material having a higher thermal coeflicient of expansion than the material of said body and said collector whereby frequency stabilization of said apparatus is achieved.

2. The apparatus as defined in claim 1 wherein a space exists between the collector and body around said frustroconical strut and including a low melting temperature alloy disposed between the collector and the body of the apparatus within said space.

3. A high frequency apparatus comprising a metal body having a bore therethrough, an electron gun assembly disposed on one end of said bore for projecting an electron beam along the axis of said bore, a collector assembly disposed at the other end of said bore, a plurality of drift tube sections disposed coaxially within said bore and forming interaction gaps, said collector assembly comprising a cylindrical member having an apertured flange at one end defining one of said drift tube sections, a frustro-conical strut having one end aflixed to the wall of said bore adjacent said flange and its other end affixed along said cylindrical member spaced from said one end, said strut being made of a metal having a different thermal coeflicient of expansion than the metal of said cylinder whereby temperature compensation of the apparatus is effected.

4. The apparatus as defined in claim 3 including, a low temperature melting alloy disposed around said frustroconical strut between said collector and said body.

5. An electron discharge device comprising electron gun and collector means disposed at opposite ends of a predetermined line of circuit development, a main body having a bore extending therethrough and operatively connected to said gun and collector means, an end wall having a drift tube segment incorporated therein, said end wall disposed within said bore and serving to partially define an interaction gap, said collector member being mounted on and aflixed to said end wall, said collector member being disposed within said bore and serving to define a space between the surface of the bore and'the external surface of the collector, temperature compensation means disposed between said body and said collector within said space, said means being adapted and arranged to control the gap spacing which is partially defined by said end wall, said body being made of a material having a lower temperature coefficient of expansion than said temperature compensation means.

6. The device as defined in claim 5 wherein said temperature compensation means is a frustro conical strut fixedly attached on the one end thereof to the body near said end Wall and fixedly attached on the other end thereof to said collector.

7. The device as defined in claim 5 wherein a loW temperature melting alloy is disposed within said space surrounding said temperature compensation means.

8. The device as defined in claim 5 wherein said body is made of a magnetically permeable material for magnetic shielding of the beam.

References Cited by the Examiner UNITED STATES PATENTS 1O GEORGE N. WESTBY, Primary Examiner. 

1. A HIGH FREQUENCY APPARATUS COMPRISING A METAL BODY HAVING A BORE THEREIN, ELECTRON GUN MEANS OPERATIVELY CONNECTED THERETO AT THE ONE END THEREOF, COLLECTOR MEANS OPERATIVELY CONNECTED THERETO AT THE OTHER END THEREOF, AN INTERACTION GAP FORMED BY OPPOSING DRIFT TUBE SECTIONS DISPOSED WITHIN SAID BORE BETWEEN SAID COLLECTOR AND GUN MEANS, AND A FRUSTO-CONICAL STRUT DISPOSED ABOUT SAID COLLECTOR AND OPERATIVELY CONNECTED TO SAID COLLECTOR AND SAID BODY, SAID FRUSTO-CONICAL STRUT BEING MADE OF A MATERIAL HAVING A HIGHER THERMAL COEFFICIENT OF EXPANSION THAN THE MATERIAL OF SAID BODY AND SAID COLLECTOR WHEREBY FREQUENCY STABILIZATION OF SAID APPARTUS IS ACHIEVED. 